Physics Chapter 11: Vibrations and Waves Chapter 12: Sound Section 12.2 Sound Intensity and Resonance 11/29/2007
Sound Intensity --Work is done on air molecules when a! vibrating object creates sound waves.! --Since work is done, energy is transferred to! the molecules; the object eventually stops! vibrating as its energy is transferred.! --The rate at which energy is transferred! through a unit area of the plane wave is the! intensity of the sound wave.!
--Since power is the rate at which energy is! transferred, the intensity of a sound wave can! also be expressed in terms of power:! intensity = ΔE Δt area = P area --The SI unit of power is the watt; thus! intensity has units of:! watts square meter = W m 2
--For a spherical wave, the power is distributed! over the surface area of sphere.! area of a sphere = 4πR 2 intensity = or intensity = P 4πr 2 (power) (4π)(distance from source) 2 --The intensity decreases as the distance from! the source increases because the same! amount of energy is spread over a larger area.!
Audible Sounds --In addition to frequency, the intensity of! sound also determines which sounds are! audible.! --Sounds with either a high or low frequency! must be relatively intense to be heard.! --Sounds with a medium frequency can be! heard at lower intensities.!
Figure 12-9, page 416!
--The lowest intensity that the average human! ear can detect is about 1.0 x 10-12 W/m 2 at! 1000 Hz; this is known as the threshold of! hearing. --The loudest sound humans can tolerate has! an intensity of 1.0 W/m 2 ; known as the! threshold of pain.
Loudness --The intensity of a sound wave determines! the loudness or volume of the sound.! --The loudness is not directly proportional to! the intensity; the sensation of sound is! approximately logarithmic in the human ear.! --Relative intensity, which relates the intensity! of a given sound to the intensity at the! threshold of hearing, is more closely related to! human perceptions of sound.!
--Relative intensity is also called the decibel level! because the unit of relative intensity is the! decibel (db). (Note that the decibel is a! dimensionless unit because it compares one! sound intensity to another.)!
Conversion of Intensity to Decibel Level (Table 12-2, page 417)!
--An increase of 10 times in the intensity adds! 10 to the decibel level.! --An increase of of ten in the decibel level! doubles the volume of the sound; the volume at! the threshold of pain is 4096 times as loud as! the volume at the threshold of hearing.!
Forced Vibrations and Resonance --When a guitar string makes the body of the! guitar vibrate, the intensity of the sound! increases; this is called a forced vibration.! (Like the strings of a piano cause the sound! board in a piano to vibrate)! --Forced vibrations are called sympathetic! vibrations.!
--The sympathetic vibrations of the guitar body! transfers the energy of the vibrating string to! the air much more quickly, increasing the! intensity of the sound produced by the guitar.! --Every object has a natural frequency of! vibration; for example, the natural frequency! of a pendulum depends upon its string length.!
--A pendulum that is vibrating will transfer its! energy to another pendulum of the same! length if they are connected, since they have! the same natural frequency; this behavior is! called resonance.! --Two tuning forks with the same frequency! can also cause each other to start vibrating!
--The Tacoma Narrows Bridge in Washington! state collapsed in 1940 when the natural! frequency of the wind produced sympathetic! vibrations in the suspension bridge; as the! vibrations became more violent the bridge! eventually collapsed.!
ISPS 07 Glass Breaking Video
Interference of Sound Waves --Sound waves may interfere with each other,! producing either constructive or destructive! interference.! --If there is a regular pattern to the interference,! the beats associated with music are produced.! --The interference of sound waves can be! observed using a frequency oscillator/amplifier! and two speakers emitting a sound of a single! frequency.!
Uses of Sound (other than speech!)! --Sonar: uses ultrasonic waves to locate! objects. (Example: bats)! --Ultrasonic cleaning: vibrations created by! ultrasounds passing through a liquid vibrate! dirt off objects placed in the liquid.!
--Ultrasound pictures: Images of body tissues! can be produced using ultrasonic waves, since! the ultrasonic waves are partially reflected! when they strike a boundary between material! of different densities. Ultrasounds can be used! to create images of internal organs or of a! fetus.!